Sublimation pump and method of regulating it

ABSTRACT

In a sublimation pump the cooled gas-adsorbing metal surface of the pump cylinder and a heated metal filament are made the anode and cathode, respectively, of a space-charged limited current diode which is included in a bridge circuit for detection of changes in its current-voltage characteristics. Potential differences across the bridge circuit resulting from changes in extent of adsorption of gas and deposition of vaporizable metal on the anode are amplified and utilized to control the source which effects heating of the vaporizable metal in the cylinder.

United States Patent Inventor Thomas A. Jennings Sutton Coldfield,England Appl. No. 17,158

Filed Mar. 6, 1970 Patented Sept. 28, 1971 Assignee Hull CorporationHatboro, Pa.

SUBLIMATION PUMP AND METHOD OF REGULATING 1T 9 Claims, 1 Drawing Fig.

US. Cl 417/51 Int. Cl F04!) 37/02 Field of Search 417/5l,48,

[56] References Cited UNITED STATES PATENTS 3,371,853 3/1968 Maliakal417/49 3,429,501 2/1969 Hamilton et a1. 417/49 Primary Examiner- RobertM. Walker Attorney-Oliver D. Olson PATENTEUSEP28|971 lif 8 r 0 r r l a U4 2 J: 4 0 4 w Thomas .4. Jennz'n 5 BY INVENTOR AQQ @mb SUBLIMATION PUMPAND METHOD OF REGULATING IT BACKGROUND OF THE INVENTION This inventionrelates to sublimation pumps and more particularly to a sublimation pumpcapable of maintaining automatically a substantially constant pumpingspeed over an adjustable range of speeds.

Sublimation pumps provided heretofore are controlled as to on" and off"time by observed variations in pump chamber pressure. This type ofcontrol depends for its accuracy and consistency upon proper attentionby operating personnel. The requirement of continuous supervision byoperating personnel burdens this type of control with excessive laborcosts, and the dependence upon human estimations and attention rendersthe accuracy and consistency of such control susceptible of significantvariation.

For example, if the off time, i.e. the time interval during which gasadsorption on the cooled metal surface is taking place, is allowed toextend for too long a period of time, excessive pump downtime isincurred, whereas if the off time is made to short, effective pumpingarea is reduced correspondingly. If the on time, i.e. the time requiredto deposit a clean metal surface over the adsorbed gas layer, is allowedto continue for too long a period of time, excessive vaporizable metalis wasted, whereas if the on time is made too short, insufficienttrapping of adsorbed gas results, with consequent reduction in effectivepumping area.

Proper on time also is dependent upon such factors as changes incomposition, geometry, temperature and operating conditions of theevaporating source, and these variations are not readily detectable asvariations in pump chamber pressure.

Furthermore, the pumping speed of such prior sublimation pumps variesconsiderably, for example by a factor of 10, between the start and endof the off time. Accordingly such pumps cannot be used in conjunctionwith a variable leak, since the latter requires a substantially constantpumping speed.

SUMMARY OF THE INVENTION In its basic concept this invention provides asublimation pump and method of regulating it, in which changes incurrent-voltage characteristics of a space-charged limited currentdiode, resulting from changes in the extent of adsorption of gas and ofdeposition of vaporizable metal, are utilized to maintain automaticallya preadjusted pumping speed.

It is by virtue of the foregoing basic concept that the principalobjective of this invention is achieved; namely to overcome theaforementioned disadvantages of prior sublimation pumps.

Another important object of this invention is the provision of asublimation pump of the class described which is of simplifiedconstruction for economical manufacture, which is precise in itsoperation and requires a minimum of maintenance and supervision.

The foregoing and other objects and advantages of this invention willappear from the following detailed description, taken in connection withthe accompanying drawing of the preferred embodiment,

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawingillustrates a sublimation pump embodying the features of this inventionand includes a cross-sectional view of the pump and a schematicelectrical diagram of control circuitry associated therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the embodiment illustratedthe pump includes a cylindrical container having a closed bottom end 12and removably sealed at its top end by a cover 14. A laterally extendingconduit 16 is adapted for connection, as by cooperating flanges, to aconduit 18 leading from a vessel (not shown) to be evacuated. Thecontainer thus defines a pumping chamber which communicates with thevessel to be evacuated.

At least the inner surface of the container, and preferably the entirecontainer, is made of stainless steel, or any other electricallyconductive metal suitable for use in high vacuum.

Means is provided for cooling the inner surface of the container so thatthe latter will adsorb such gases as oxygen, nitrogen, hydrogen,ammonia, carbon monoxide, carbon dioxide, sulfur dioxide,tetracyanoethylene, water vapor and many others. In the embodimentillustrated such means is provided by a vessel 20 made in the form of aDewar flask. The top of the vessel is open to receive the cylinder whichthus may be immersed in a liquid coolant 22 contained in the vessel. Thecoolant may be water, liquid nitrogen or other suitable form of liquid.It will be understood that various other well-known means may beutilized to effect cooling of the inner surface of the container.

Within the container there is disposed a source 24 of active metal, i.e.metal capable of adsorbing the desired gas particles, The metal also iscapable of being vaporized for deposition in film form on the innersurface of the container. Although titanium is most commonly used forthis purpose, molybdenum, tungsten, tantalum, zirconium, chromium,barium and various other metals also may be employed, as will beunderstood. Evaporation of the metal may be effected by various means,such as an electron beam source. However, in the embodiment illustratedthe vaporizable metal source 24 is provided in the form of a filamentrod which is supported by electrodes 26 and 28. These extend from thecover 14 and are connected to terminals 26' and 28'. It will beunderstood that the electrodes and terminals are insulated electricallyfrom the cover, if the latter is made of electrically conductivematerial. The terminals are connected through electrical conductors 30and 32 to a power supply circuit 34.

In the power supply circuit the conductors 30 and 32 are connected,preferably through an ammeter 36, across the secondary winding of astepdown transformer 38. One end of the primary winding of thetransformer is grounded, while the opposite end is connected to thevariable contact of a Variac '40. One end of the Variac coil isgrounded, while the opposite end is connected to one end of thesecondary winding of the transformer 42. The opposite end of thissecondary winding is connected to ground selectively by means of a pairof switches which, in the embodiment illustrated, are in the form ofoppositely arranged silicon controlled rectifiers 44 and 46. Eachsilicon controlled rectifier conducts during opposite half cycles of analternating current signal supplied from a signal source described indetail hereinafter. Only one silicon controlled rectifier need beprovided, if operation during one-half cycle only is desired. Thesilicon controlled rectifiers may be bypassed by a switch 48 for manualoperation of the power supply. The primary winding of transformer 42 isconnected through the control switch 50 to the terminals 52 of a sourceof electric potential.

Surrounding the vaporizable metal rod 24 is a cathode filament 54 oftungsten or other suitable metal. In the embodiment illustrated thecathode filament is in the form of a coil, although it will be apparentthat other configurations may be selected as desired. The cathodefilament is connected at its opposite ends to electrodes 56 and 58 whichare supported by the cover 14. The electrodes are connected throughterminals 56' and 58 to electrical conductors 60 and 62. Battery 64provides an isolated power supply for heating the cathode filament toelectron emission temperature, and it is connected across the conductorsthrough a control switch 66 and variable resistance 68. The latterserves to adjust the cathode temperature such that the diode current isspace-charged limited, and also to prevent deposition of metal vapors onthe filament.

The inner surface of the container 10 and the cathode filament 54 aremade the anode and cathode, respectively, of a space-charged limitedcurrent diode, and means is provided for detecting variations incurrent-voltage characteristics thereof. To this end, and in theembodiment illustrated, the electrically conductive container isconnected through the electrical conductor 70 to the grounded junction72 of a bridge circuit. One end of the cathode filament 54 is connectedthrough the electrode 58 and conductor 62 to a junction 74 of the bridgecircuit which forms with the grounded junction 72 one branch of thebridge circuit. The other branches of the bridge circuit includevariable reference resistance 76 and fixed resistances 78 and 80. Thejunctions 74 and 82 of the bridge circuit are interconnected through theseries arrangement of control switch 84, direct current constant voltagebridge power supply 86, connected in the polarity indicated, and ammeter88.

The output junction 90 of the bridge circuit is connected to the inputof a direct current differential amplifier 92 through control switch 94and diode 96, and preferably also through ganged selector switches 98and 100 which accommodate operation with either electronegative orelectropositive gases. Thus, operation with electronegative gases isafforded with the selector switches arranged in the positionsillustrated. when these switches are transferred to their alternatepositions, the signal from the bridge is fed to the inverting input ofthe operational amplifier 102. The noninventing input is connected toground. With resistors 104 and 106 being equal, the bridge input isamplified by the factor i, and therefore the input and output voltagesare equal and only the polarity is reversed.

The output of the amplifier 92 is connected through conductor 108 to thesilicon controlled rectifiers 44 and 46. It will be apparent thatvarious vacuum tube of solid state type differential amplifiers may besubstituted for the one which is illustrated in the drawing merely forpurposes of explanation.

This invention utilizes the current-voltage characteristics of aspace-charge limited current diode to select and maintain a desiredsublimation pumping speed.

The space-charged limited current density J, in amperes per squaremeter, may be expressed in terms of the potential difference V betweenthe cathode and anode, in volts, by the equation J'=2.33 10 (V) "*ld"wherein d is the distance between the cathode and anode, in meters.

Since the space-charged limited current is more dependent upon theapplied cathode to anode voltage and the anode electron work functionthat on the cathode electron work function, the potential difference Vcan be expressed as the sum of the applied cathode to anodg voltage V,and the average anode electron work function 0, in volts.

The electron work function is defined as the energy required to removean electron in the Fermi energy state to a point outside the metal andbeyond the influence of its electric fields. Adsorbed gas layers thencan either increase or decrease the apparent electron work function,depending upon whether the adsorbed gas is electronegative orelectropositive. To increase the apparent electron work function theadsorbed gas layer must be electronegative. Examples of such gases areoxygen and nitrogen. The very high electric fields existing between themetal anode surface and the negatively charged adsorbed layer retardelectrons from leaving the surface, thereby increasing the work functionof the surface.

An absorbed electropositive gas layer examples of which are ammonia andtetracyanoethylene, also generate high electric fields. However, thesefields lower the energy required for the electrons to leave the surfaceand thus result in decreasing the apparent electron work function.

An increase in the average work function requires that the appliedvoltage V, must be increased in order to maintain a constant currentdensity J. On the other hand, a gas layer that decreases the averageelectron work function requires that the applied voltage be reduced inorder to maintain a constant current density.

The average anode electron work function may be expressed by theequation wherein 0,, is the electron work function of the clean metalanode surface, fl, is fraction of the unit surface area free of anadsorbed gas layer, and jiis the fraction of the anode surface that hasa work function Q,,- Thus, if Brepresents the fraction of the unitsurface area which is free of an adsorbed gas layer and 1-8) is setequal to Efl-Q then 9 min QB wherein Q is the electron work function forthe adsorbed gas layer, in volts, and

The total sublimation pumping speed S for a gaseous mixture at a givenpressure and active metal film temperature is defined by the equation S=9AZx,S wherein A is the total cooled surface area of the anode, insquare meters; 1:, the ratio of partial pressure of the gas component ito the total pressure; and S, is the pumping speed per unit area of thewith gas component.

The last two equations may be combined into the equation (Org 5;; (Kwhich shows that the total pumping speed of the sublimation pump can beexpressed as a function of the applied cathode to anode voltage and thespace-charges limited current density. Changes in the pump diodecharacteristics are detected by use of the bridge circuit previouslydescribed.

Thus, with the space-charged limited current density varying with thecathode to anode voltage and the anode electron work function, thepotential difference across the bridge circuit is related to the extentof gas adsorption on the anode and the extent of deposition of vaporizedmetal film on the anode.

initially, with the bridge balanced, the current through each of thebranches is equal and potential difference between the points and 72 iszero. When the adsorption of an electronegative gas layer covers apredetermined area of the anode, the reduction in current of the spacecharged limited current diode produces a potential difference across thebridge which provides an electric signal output from the bridge ofsufficient magnitude, when amplified by the different amplifier 92, tofire the silicon controlled rectifiers 44 and 46. Current thus isapplied from the power supply 34 to the vaporizable metal rod 24.

In similar manner, when the adsorption of an electropositive gas layercovers a predetermined area of the anode, the increase in current of thespace-charged limited current diode produces a potential differenceacross the bridge. The resulting electric signal output from the bridgeis fed to the operational amplifier M2, by transferring the switches 98,to the alternate positions as previously explained. lf this signaloutput is of sufficient magnitude, when amplified by differentialamplifier 92, to fire the silicon controlled rectifiers 44 and 46,current will be supplied to the vaporizable metal rod 24.

When the deposition of vaporizable metal film covers a predeterminedarea of the anode, the amplified bridge output voltage can no longerfire the appropriate silicon controlled rectifier. Accordingly thesource of current for the vaporizable metal rod 24 is cut off and metalvapor deposition is stopped. Adsorption of a next subsequent gas layeron the clean anode surface then proceeds.

The pump also may be adjusted to increase or decrease the pumping speed.It has been shown hereinbefore that the pumping speed is proportional tothe fraction of the area of the anode not covered by an adsorbed gaslayer and that the latter is a function of the diode current density.

Accordingly, let it be assumed that the bridge voltage across thejunctions 90 and 72 is not sufficient, when amplified, to fire thesilicon control rectifier and that the pumping speed is S In order toincrease the pumping speed to some value S the diode current density 1.,must be increased to a valve 1, This is accomplished by decreasing theresistance of the reference resistor 76 to increase the potentialdifference across the junctions 90 and 72 to a magnitude which, whenamplified, effects firing of the silicon controlled rectifiers. Theresulting deposition of metal vapors results in an increase in thefraction of the anode area not covered by the adsorbed gas layer and acorresponding increase in the pumping speed. When the desired pumpingspeed S, is reached, the amplified bridge output voltage can no longerfire the silicon controlled rectifier and metal vapor deposition isstopped.

To reduce the pumping speed from S x to S the resistance of thereference resistor 76 is increased. This increases the magnitude of thebridge output voltage, but the polarity at junction 90 is reversed fromnegative to positive with respect to the potential at junction 72. Thediode 96 functions to prevent large grid currents when the grid ispositive with respect to the cathode of the input tube of the amplifier92. The amplified output from the amplifier is insufficient to effectfiring of the silicon controlled rectifiers.

When sufficient gas particles have been adsorbed on the anode surface,the pumping speed will be reduced from the valve of S I to S, and thecurrent through the reference resistor 76 will be equal to the totalspace-charged limited current diode. Further adsorption of gas causes anegative output voltage from the bridge sufficient to effect firing ofthe silicon controlled rectifiers and consequent vapor deposition ofmetal on the anode.

To illustrate the foregoing, let it be assumed that the surface area ofthe anode is 0.081 square meter, the cathode to anode distance is 0.038meter, the electron work functions for clean titanium is 5.8 volts, theelectron work function for an adsorbed oxygen layer is 6.8 volts, andthe maximum diode current density at a maximum sublimation pumping speedof 1,215 liters per second is 0.015 amperes per square meter, providingcathode to anode voltage of 10.24 volts. Assuming that at maximumpumping speed all of the resistance values in the bridge circuit areequal, the total voltage applied across the bridge is 20.48 volts andeach of the resistors is 8,450 ohms. Power supply 86 supplies 36 voltsand the differential amplifier provides an amplification of 1,000.

The operation of the sublimation pump is as follows: First pressure inthe vessel to be evacuated, and hence in container 10, is reduced,either by a mechanical pump or an adsorption pump, to about i XlOtorr.Liquid nitrogen then is filled into the vessel 20 to cool the anodesurface.

The differential amplifier 92 now is adjusted by first closing theswitches 110, 112 and 114. The potential difference across the resistor116 is fixed at 0.005 volts and thus by means of the variable resistors118 and 120 the output voltage of the amplifier is adjusted to fivevolts, as measured on the voltmeter 122. The switches then are opened.

The bridge circuit now is adjusted first by setting the referenceresistor 76 to a low pumping speed. With switches 98 and 100 in thepositions illustrated, for operation with electronegative oxygen gas,switches 66, 84 and 94 then are closed and the resistance of thevariable resistor 68 is decreased until further reduction in resistancedoes not increase the current measured by the meter 88.

The switch 110 in the amplifier circuit now is closed. The voltmeter 122will indicate the amplifier output to be about 9 to 10 volts. Thetitanium filament power supply circuit 34 now is activated by closing eswitch 50 to provide filament current of, for example, not more thanabout 50 amperes. The variac 40 now is adjusted slowly to increase thefilament current until it suddenly decreases to zero. This occursdeposition of vaporized titanium film on the cooled anode surface.

The reference resistor 76 of the bridge circuit now is adjusted slowlytoward the desired pumping speed, care being taken to limit the outputvoltage of the amplifier to not more than about 11 volts. The rate atwhich the output voltage of the amplifier decreases is controlled by thevariac 40.

When the desired pumping speed has been attained, it will be maintainedsubstantially constant, provided there is suffcient titanium forevaporation and the proper coolant level is maintained in the vessel 20.

The pumping speed may be adjusted over a range by varying the cathode toanode voltage, by adjusting the reference resistor 76, as previouslyexplained. Although the diode current density may be varied, care shouldbe taken to limit its maximum value to prevent desorption of gas fromthe cooled surface due to electron impact.

The foregoing operation of trapping successive layers of adsorbed gas bysuccessive films of vaporized metal, continues until the pressure in thevessel being evacuated reaches a desired value.

The sublimation pump is turned 05 by first opening the switch 50 in thefilament power supply 34 and then opening the switches 110, 84 and 66 inthat order.

It has been determined that in the forgoing illustration the pumpingspeed will decrease about 10 percent of the maximum possible pumpingspeed before the silicon controlled rectifier will fire to effect vapordeposition of active metal on the anode. Sublimation pumps of the priorart exhibit pumping speed fluctuations approaching percent of themaximum pumping speed. Accordingly, the significant improvement of thepump of this invention is self evident. On the other hand, still furtherreduction in the pumping speed fluctuation of the pump of thisinvention, to within about 1 percent of the maximum pumping speed, canbe achieved by correcting for the bridge voltage necessary to fire thesilicon controlled rectifier. The power level of the vaporizing metalfilament 24 thus at a minimum, with corresponding increase in its usefullife.

it will be apparent to the skilled in the art that various changes maybe made in the size, shape, number, type and arrangement of partsdescribed hereinbefore without departing from the spirit of thisinvention.

Having now described my invention and the manner in which it may beused, I claim:

k. A sublimation pump comprising a. a container having an inner surfaceof electrically conductive metal and defining a chamber arranged forcommunication with a vessel to be evacuated,

b. cooling means in contact with the container for cooling the innersurface thereof,

c. vaporizable metal in the chamber,

d. means for heating the vaporizable metal to vaporize the latter fordeposition on the inner surface of the container,

e. electrical control means for said heating means,

f. metal filament means in the chamber,

g. means for heating the filament means to electron emissiontemperature,

. electric circuit means connected to the inner surfaces of thecontainer and to the filament means and forming said inner surface andfilament means as the anode and cathode, respectively, of aspace-charged limited current diode,

i. electric signal producing means having an input connected to saidelectric circuit means and operable upon changes in the current-voltagecharacteristics of the diode to produce an electric output signal, and

j. means connecting the output of the signal producing means to theelectrical control means for activating the vaporizable metal heatingmeans when the electrical output signal reaches a predetermined value.

2. The sublimation pump of claim 1 wherein the signal producing meansincludes an electrical bridge circuit in which the diode forms onebranch, and an electric signal amplifier has its input connected to theoutput of the bridge circuit and its output connected to the electricalcontrol means.

3. The sublimation pump of claim 2 wherein the control means compriseselectric switch means operable by the electric output signal of theamplifier.

4. The sublimation pump of claim 3 wherein the electric switch meanscomprises a silicon controlled rectifier.

5. The sublimation pump of claim 1 wherein a. the vaporizable metal isan elongated rod,

b. the heating means includes an electric power supply connected to therod,

c. the filament means is a coil surrounding the rod,

d. the signal producing means includes an electric bridge circuit inwhich the diode forms one branch, and an electric signal amplifier hasits input connected to the output of the bridge circuit and its outputconnected to the electrical control means, and

e. the control means comprises a silicon controlled rectifier in thecircuit of the power supply operable upon activation by the electricoutput signal of the amplifier to activate the power supply.

6. The sublimation pump of claim 1 wherein the vaporizable metal is anelongated rod, the heating means includes an electric power supplyconnected to the rod, and the filament means is a coil surrounding therod.

7. The sublimation pump of claim 6 wherein the control means compriseselectric switch means in the circuit of the power supply operable by theelectric output signal of the signal producing means.

8. in a sublimation pump including a container having an inner surfaceof electrically conductive metal and defining a chamber arranged forcommunication with a vessel to be evacuated, cooling means in contactwith the container for cooling the inner surface thereof, vaporizablemetal in the chamber, means for heating the vaporizable metal tovaporize the latter for deposition on the inner surface of thecontainer, and electrical control means for said heating means, themethod of controlling automatically the adsorption of gas and depositionof vaporizable metal on said cooled surface, comprising a. placing acathode filament in the chamber,

b. forming an electric circuit in which the inner surface of thecontainer and the cathode filament are made the anode and cathode,respectively, of a space-charged limited current diode, and

c. utilizing the changes in current-voltage characteristics of the dioderesulting from adsorption of gas and deposition of vaporizable metal onsaid cooled surface to control the heating of the vaporizable metal.

9. The method of claim 8 including the steps of utilizing the changes incurrent-voltage characteristics of the diode to produce an electricsignal, amplifying said electric signal, and utilizing said amplifiedelectric signal to operate the electrical control means for said heatingmeans.

1. A sublimation pump comprising a. a container having an inner surfaceof electrically conductive metal and defining a chamber arranged forcommunication with a vessel to be evacuated, b. cooling means in contactwith the container for cooling the inner surface thereof, c. vaporizablemetal in the chamber, d. means for heating the vaporizable metal tovaporize the latter for deposition on the inner surface of thecontainer, e. electrical control means for said heating means, f. metalfilament means in the chamber, g. means for heating the filament meansto electron emission temperature, h. electric circuit means connected tothe inner surface of the container and to the filament means and formingsaid inner surface and filament means as the anode and cathode,respectively, of a space-charged limited current diode, i. electricsignal producing means having an input connected to said electriccircuit means and operable upon changes in the current-voltagecharacteristics of the diode to produce an electric output signal, andj. means connecting the output of the signal producing means to theelectrical control means for activating the vaporizable metal heatingmeans when the electric output signal reaches a predetermined value. 2.The sublimation pump of claim 1 wherein the signal producing meansincludes an electrical bridge circuit in which the diode forms onebranch, and an electric signal amplifier has its input connected to theoutput of the bridge circuit and its output connected to the electricalcontrol means.
 3. The sublimation pump of claim 2 wherein the controlmeans comprises electric switch means operable by the electric outputsignal of the amplifier.
 4. The sublimation pump of claim 3 wherein theelectric switch means comprises a silicon controlled rectifier.
 5. Thesublimation pump of claim 1 wherein a. the vaporizable metal is anelongated rod, b. the heating means includes an electric power supplyconnected to the rod, c. the filament means is a coil surrounding therod, d. the signal producing means includes an electric bridge circuitin which the diode forms one branch, and an electric signal amplifierhas its input connected to the output of the bridge circuit and itsoutput connected to the electrical control means, and e. the controlmeans comprises a silicon controlled rectifier in the circuit of thepower supply operable upon activation by the electric output signal ofthe amplifier to activate the power supply.
 6. The sublimation pump ofclaim 1 wherein the vaporizable metal is an elongated rod, the heatingmeans includes an electric power supply connected to the rod, and thefilament means is a coil surrounding the rod.
 7. The sublimation pump ofclaim 6 wherein the control means comprises electric switch means in thecircuit of the power supply operable by the electric output signal ofthe signal producing means.
 8. In a sublimation pump including acontainer having an inner surface of electrically conductive metal anddefining a chamber arranged for communication with a vessel to beevacuated, cooling means in contact with the container for cooling theinner surface thereof, vaporizable metal in the chamber, means forheating the vaporizable metal to vaporize the latter for deposition onthe inner surface of the container, and electrical control means forsaid heating means, the method of controlling automatically theadsorption of gas and deposition of vaporizable metal on said cooledsurface, comprising a. placing a cathode filament in the chamber, b.forming an electric circuit in which the inner surface of the containerand the cathode filament are made the anode and cathode, respectively,of a space-charged limited current diode, and c. utilizing the changesin current-voltage characteristics of the diode resulting fromadsorption of gas and deposition of vaporizable metal on said cooledsurface to control the heating of the vaporizable metal.
 9. The methodof claim 8 including the steps of utilizing the changes incurrent-voltage characteristics of the diode to produce an electricsignal, amplifying said electric signal, and utilizing said amplifiedelectric signal to operate the electrical control means for said heatingmeans.